The appliances used for levigation may be of a very simple character, consisting only of several tubs or tanks, mounted in such a way that the liquid contained in one can be run off into the one next below. With this primitive plant, the material to be levigated is stirred up in the water in the uppermost tub and left to settle until the coarsest particles may be assumed to have settled down, whereupon the turbid water is drawn off into another tub, in which it is left to settle completely. When the clear liquid has been carefully drawn off, a fine sludge is left in the bottom of the tub, consisting of the fine particles of material mixed with water. When a particularly fine powder is required, a single levigation does not always suffice, but the liquid in the second tub must be left to settle for a short time only, and then run into a third for complete subsidence. [Illustration: FIG. 14.] A well-designed levigator for treating large quantities of powder is illustrated in Fig. 14. A stirrer _R_, driven by cone gearing, is arranged in a wooden or stone vat _G_. The levigating water enters close to the bottom of the vat, through the pipe _W_. When _G_ is half full of water, the stirrer is set running, and the substance to be levigated is added. After a while, the water laden with the levigated powder begins to run off at _A_ into the long narrow trough _T_{1} provided, at the opposite end from _A_, with a number of perforations through which the water runs into the trough _T_{2}. From this it escapes through the perforations into the trough _T_{3} and thence successively into _T_{4} and _T_{5}, finally discharging into the large tank _S_. The coarsest and heaviest of the water-borne particles deposit in the trough _T_{1} finer particles settling down in _T_{2}, and so on in succession, until the water reaching the tank _S_ contains only the very finest of all in suspension, these taking a long time to settle down to the bottom. The deposit in the upper troughs can be returned to the vat, whilst that in the lower ones will be fine enough to dry as it is. The residue in the vat is discharged through _Z_ when the operation is finished. It will be evident that the fineness of the product depends on the number and length of the troughs _T_, the larger these factors the more delicate will be the particles remaining in prolonged suspension in the liquid.